In recent years, by means of the temperature modulation effect of optical fiber Raman scattering light intensities and the optical time domain reflection (OTDR) principle, distributed optical fiber Raman temperature sensors have been developed. They can be used to measure the on-site temperature in real time, predict the temperature change trend, monitor the on-site temperature change and provide on-line temperature alarming when the measured temperature is over a certain range. The distributed optical fiber Raman temperature sensor is a linear temperature response detector of safe type in nature and easy to build an optical fiber sensor network. It has been successfully applied in the fields of power industry, petrochemical enterprises, large scale civil engineering, on-line disaster monitoring, etc.
The optical fiber Raman scattering frequency shift is about 13.2 THz. So there is a relatively large wavelength difference between the anti-Stokes Raman scattering light and the Stokes Raman scattering light of the optical fiber. Due to the dispersion effect in optical fibers, the backward anti-Stokes Raman scattering light and the Stokes Raman scattering light have different transmission velocities in the optical fiber, thus leading to the “asynchronism” or “separation” phenomenon between the anti-Stokes Raman scattering light and the Stokes Raman scattering light in the time domain reflection curves. For distributed optical fiber Raman temperature sensors, the time domain reflection signal of the optical fiber backward Stokes Raman scattering light is used to demodulate the time domain reflection signal of the anti-Stokes Raman scattering light, in order to obtain temperature information of all segments of the optical fiber. However, the “asynchronism” or “separation” phenomenon happened in the two OTDR signals decreases the spatial resolution and temperature measurement precision of the sensor system, and even causes measurement errors. In a distributed optical fiber Raman temperature sensor, the anti-Stokes Raman scattering light is used as the temperature measurement signal channel, while the Stokes Raman scattering light is used as the temperature measurement reference channel. Since the two channels have quite different wavelength and the optical fiber attenuation losses are different for various wavelengths, they have different intensity losses. Therefore, when the Stokes Raman reference channel is used to demodulate the anti-Stokes Raman signal, the demodulated temperature curve presents a non-linear feature. This causes the temperature measurement errors and decreases the temperature measurement precision. Additionally, when optical fibers are installed on-site, they are very likely bent and stretched which leads to non-linear optical effects in optical fibers, and causes power losses at different wavelengths. Furthermore, since both the magnitude and the position of the bend and the stretch in pressing of the optical fiber and optical cable are random and unpredictable, it is difficult to correct the measurement manually. Thus, an auto-correction method is needed.
In 2007, Chung E. Lee et al proposed a solution: “Methods and Apparatus for Dual Source Calibration for Distributed Temperature Systems” which has been granted a U.S. patent right (No. US2007/0223556A1), wherein dual light sources are employed, and the optical fiber backward anti-Stokes Raman scattering wave of the primary laser and the optical fiber Stokes Raman scattering wave of the secondary laser within the same waveband are alternately controlled by an optical fiber switch in a manner of time-division. The optical fiber backward anti-Stokes Raman scattering wave of the primary laser is demodulated by the optical fiber Stokes Raman scattering wave of the secondary laser so as to obtain the temperature information of all segments of the optical fiber. In such round trip Raman scattering OTDR signals, though the return thereof fall within the same waveband, the wavelength of the incident waves lies in the primary laser wavelength and the secondary laser wavelength which differ from each other by a dual Raman shift, so that the influences of the optical fiber dispersion spectrum and the optical fiber loss spectrum cannot be eliminated completely.